The present invention relates in general to control valves for fluid power systems, and, more specifically, to a double valve for simultaneously achieving a high input to output flow coefficient during its actuated state and a very low outlet pressure during a faulted state.
Machine tools of various types operate through a valving system, which interacts with a fluid power-controlled clutch and/or brake assembly. The control valves used to operate these machine tools require the operator to activate two separate control switches substantially simultaneously to ensure that an operator's hands or other obstructions are away from the moving components of the machine tool when an operating cycle is initiated. Typically, an electronic circuit responsive to the two control switches generates a control signal applied to the actuators for switching the main fluid circuit of the valve to control delivery of compressed air or other fluid to the machine tool to perform its operating cycle.
Double valves having two separate valve units operating in parallel within one main valve body have been developed to ensure that a repeat or overrun of a machine tool operating cycle cannot be caused by malfunction of a single valve unit. For example, if one valve unit fails to deactuate at the proper time or if the valve units actuate in a non-synchronous manner, the double valve assumes a fault configuration that continuously diverts the source of fluid power away from the machine tool. A double valve is shown, for example, in commonly assigned U.S. Pat. No. 6,840,258 and U.S. Pat. No. 6,840,259, which are incorporated herein by reference.
In known double valves for operating presses and other machine tools, it is desirable to quickly exhaust the pressurized fluid from the outlet when the valve is deactuated so that the outlet pressure rapidly drops to the pressure that allows the brake mechanism to actuate. The coefficient of flow, Cv, is a measure of a device's efficiency in permitting fluid flow, and is calculated based on measured fluid flow rate and the pressure differential across an orifice. The Cv measured in a double valve from the inlet to the outlet is not typically equal to the Cv measured from the outlet to the exhaust. A double valve utilized in press applications typically has a higher outlet-to-exhaust Cv than an inlet-to-outlet Cv.
Fluid flow through crossover passages within the double valve cause the movement of one valve unit to influence the movement of the other valve unit. The crossover passages are normally pressurized in both the actuated and deactuated states of the valve. In a faulted state of the valve, one crossover is pressurized and the other is opened to the outlet and to the exhaust port through the outlet. Inlet pressure flowing into the crossover that is open to the outlet causes a certain amount of pressure to continue to be present in the outlet during the faulted state. Industry standards state that such pressure should be maintained at less than one percent of the pressure of the fluid supply. Due to the relatively large flow capacity of the exhaust, prior art double valves for press and machine tool applications met the objective of less than one percent.
It would be desirable to utilize the lockout capability and dynamic monitoring features of double valves in non-press applications. Such applications would typically employ a continuous and relatively greater Cv between the inlet and the outlet, which is achieved by scaling up the sizes of the inlet, outlet, and valve units. The scaling up of the inlet-to-outlet Cv, however, would tend to increase the flow into the outlet during a faulted state which is undesirable for the reasons stated above. This increase is due in part to the pressurization of the crossover passages through respective flow restrictors provided between the crossover passages and the inlet. The flow restrictors are also part of the main flow paths between the inlet and the outlet. When the inlet, outlet, and valve units are scaled up to provide a higher Cv, the flow restrictor passages feeding the crossovers likewise are scaled up so that it is not possible to effectively limit or control the flow from the crossover into the outlet during a fault state by scaling these various valve elements.
Maintaining an outlet pressure below one percent of inlet pressure could be achieved by scaling up of the exhaust port and the exhaust poppets, but these steps are undesirable because of added cost and an increased-package size for the double valve.